The present invention relates to a rubber-modified styrenic resin composition which provides a molded article which is excellent in plane impact strength and gloss, and a molded article thereof. In particular, the present invention relates to a rubber-modified styrenic resin composition which provides a molded article plane impact strength of which is greatly increased, and gloss of which is excellent by the addition of a specific polymer to a specific rubber-modified styrenic resin, and a molded article thereof.
In the fields of office automation equipments and household appliances, good balance of various properties such as processability in a molding process, accuracy of finished sizes of processed products, mechanical properties (e.g. tensile strength, flexural strength, etc.), heat resistance, and so on is required. In particular, when a resin is used as an exterior material, further improvement of gloss and plane impact resistance is required. In these years, such requirement reaches to a very high level. When the resin is used as a wrapping material, high level plane impact strength and appearance are required. When the resin is used as a cushioning material, one of the essential properties which the resin should have is good shock absorbability. However, the rubber-modified styrenic resin does not necessarily satisfy all the above requirements.
As a method for improving the impact resistance of the rubber-modified styrenic resin, there is known the addition of specific amounts of an organic polysiloxane and an ethylene-unsaturated carboxylate copolymer to the rubber-modified styrenic resin. However, the dispersed rubber particles have an average particle size of 0.1 to 2.5 xcexcm, and the plane impact strength of the composition is insufficient.
An object of the present invention is to solve the above problems and provide a rubber-modified styrenic resin composition excellent in the properties which are required for the exterior materials, wrapping materials and molded foam articles, for example, excellent in plane impact strength and gloss, as well as molding properties and other physical properties.
According to a first aspect of the present invention, there is provided a rubber-modified styrenic resin composition comprising (A) 100 wt. parts of a rubber-modified styrenic resin which contains 10 to 35 wt. % of soft component particles having an average particle size of 0.1 to 0.5 xcexcm, where said soft component particles have a single occlusion structure comprising a core part which consists of a single continuous phase of a styrenic resin and a shell part which comprises a rubber polymer and occludes said core part, and (B) 0.1 to 10 wt. parts of a polymer having a solubility parameter (SP) of 8.45 to 8.70 and comprising no aromatic vinyl compound unit therein.
According to a second aspect of the present invention, there is provided an injection molded article, an extrusion molded article or a molded foam article comprising the above rubber-modified styrenic resin composition.
The rubber-modified styrenic resin (A) used in the present invention may be a rubber-modified styrenic resin which is obtained by polymerizing at least one styrenic monomer, or at least one styrenic monomer and a compound copolymerizable therewith in the presence of a rubber polymer
Examples of the styrenic monomer which is used as a raw material of the rubber-modified styrenic resin (A) used in the present invention are styrene, xcex1-alkyl-substituted styrenes such as xcex1-methylstyrene, nucleus substituted alkylstyrenes such as p-methylstyrene, and so on. Examples of the compound copolymerizable with the styrenic monomer are vinyl monomers such as acrylonitrile, methacrylonitrile, methacrylic acid, methyl methacrylate, etc., maleic anhydride, maleimide, nucleus substituted maleimide, and so on.
As the rubber polymer, polybutadiene, styrene-butadiene copolymers, ethylene-propylene-non-conjugated diene terpolymers, and so on are used. Among them, polybutadiene and the styrene-butadiene copolymers are preferred. As the polybutadiene, high-cis polybutadiene having a high cis-structure content and low-cis polybutadiene having a low cis-structure content are both used.
A content of the soft component particles in the rubber-modified styrenic resin (A) is from 10 to 35 wt. %. When this particle content is less than 10 wt. %, the plane impact strength is not sufficiently improved, while it exceeds 35 wt. %, properties other than the plane impact strength, for example, stiffness, heat resistance, etc. are deteriorated unpreferably.
A content of the soft component particles in the rubber-modified styrenic resin (A) is measured as follows:
About 0.5 g of a sample of the rubber-modified styrenic resin is weighed (weight: W1), and the sample is dissolved in a mixed solvent of methyl ethyl ketone and methanol (a volume ratio of 10/1) (50 ml) at room temperature (about 23xc2x0 C.). Then, undissolved components are isolated by centrifugation, dried, and then weighed (weight: W2). A content of the soft component particles in the rubber-modified styrenic resin is calculated by the equation:
(W2/W1)xc3x97100(%).
An average particle size of the soft component particles are from 0.1 to 0.5 xcexcm, preferably from 0.1 to 0.3 xcexcm. When the average particle size is less than 0.1 xcexcm, the plane impact strength of the molded article is not sufficiently improved, while when it exceeds 0.5 xcexcm, the appearance of the molded article such as gloss is deteriorated unpreferably.
Herein, the average particle size is defined as follows:
A very thin section of the rubber-modified styrenic resin is prepared, and its transmission electron microscopic photograph is taken. Particle sizes of the soft component particles in the photograph are measured and the average particle size is calculated by the following equation:
Average particle size=xcexa3niDi2/xcexa3niDi
wherein Di is a particle size, and ni is the number of the particles having the particle size Di.
In the present invention, the soft component particles in the rubber-modified styrenic resin (A) should have a single occlusion structure comprising a core part which consists of a single continuous, phase of a styrenic resin and a shell part which comprises a rubber polymer and occludes said core part, which structure may be referred to as a core-shell structure or a capsule structure. The gloss is deteriorated when the particles have other structure, for example, a salami-like structure in which plural minute particles of the styrenic resin are dispersed in a continuous phase of the rubbery polymer. The structure of the soft component particles is observed with a transmission electron microscope as in the above measurement of the average particle size.
The synthesis of the rubber-modified styrenic resin (A) in which the structure of the soft component particles is the single occlusion structure is described in, for example, Die Angewandte Macromolekulare Chemie, 58/59, 175-198 (1977), and such resin can be synthesized by polymerizing a styrenic monomer in the presence of a styrene-butadiene block copolymer having a styrene content of 15 to 65 wt. %.
According to the present invention, the composition contains 0.1 to 10 wt. parts of a polymer (B) having a solubility parameter (SP) of 8.45 to 8.70 and comprising no aromatic vinyl compound unit therein per 100 wt. parts of the rubber-modified styrenic resin (A). When the content of the polymer (B) is less than 0.1 wt. part, the plane impact strength is not sufficiently improved, while when it exceeds 10 wt. parts, other physical properties such as heat resistance are deteriorated unpreferably.
The content of the polymer (B) in the composition of the present invention can be obtained by preparing a very thin section of the composition, taking a transmission electron microscopic photograph of the slice piece, calculating an a percentage of areas of the polymer (B) in the whole photographic area, and converting the percentage to the content of the polymer (B). Alternatively, a method using a spectrometer such as a NMR spectrometer or an IR spectrometer and calculating the content of the polymer (B) from an absorption peak, or a method comprising fractionating the polymers with a solvent may be used.
The solubility parameter (SP) of the polymer (B) used in the present invention is from 8.45 to 8.70. When the solubility parameter is larger than 8.45 or smaller than 8.70, the improvement of the plane impact strength is insufficient.
Herein, the solubility parameter is defined as an attraction force between molecules according to the Hildebrand-Scatchard theory. This theory is described in xe2x80x9cThe Solubility of Nonelectrolytesxe2x80x9d, Third Edition, Reinhold Publishing Corp., New York, 1949, and Chem. Rev., 8, 321 (1931), as well as common textbooks in the polymer chemistry. The solubility parameter can be experimentally measured by a viscosity method or a swelling method, or calculated from molecular structures, and values of the solubility parameter differ slightly depending on the methods. Herein, there is used a method for calculating the solubility parameter from the molecular structures which is proposed by Small. This method and theory are described in detail in J. Appl. Chem., 3, 71-80 (1953), the disclosure of which is hereby incorporated by reference. According to this literature, the solubility parameter is calculated by the following equation:   SP  =                              ∑                      xe2x80x83                          ⁢                  xe2x80x83                ⁢                  F          i                    V        =                  ρ        xc3x97                              ∑                          xe2x80x83                                ⁢                      xe2x80x83                    ⁢                      F            i                              M      
wherein Fi is a molar attracting force of a structural group which constitutes a molecule such as an atom, an atomic group or a bond type, V is a molar volume, xcfx81 is a density, and M is a molecular weight of a compound, or a molecular weight of one repeating unit (namely a monomer) in the case of a polymer molecule. As the Fi values, those of Small described in the above two literatures are used. Each of xcfx81, xcexa3Fi and M of a copolymer is calculated as a sum of a product of xcfx81, xcexa3Fi or M of a homopolymer of each monomer and a molar percentage of the respective monomer.
The polymer (B) used in the present invention is a polymer which has no aromatic vinyl compound unit therein. When the resin composition contains a polymer which comprises an aromatic vinyl compound unit, the plane impact strength is low.
Examples of the aromatic vinyl compound are styrene, xcex1-alkyl-substituted styrenes such as xcex1-methylstyrene, nucleus substituted alkylstyrenes such as p-methylstyrene, and so on.
Examples of the polymer (B) having the solubility parameter of 8.45 to 8.70 are copolymers comprising ethylene and at least one vinyl monomer selected from the group consisting of unsaturated carboxylic acids, unsaturated carboxylates and vinyl acetate. Specific examples of such copolymer are ethylene-unsaturated carboxylic acid copolymers, ethylene-unsaturated carboxylate copolymers, ethylene-vinyl acetate copolymers, ethylene-unsaturated carboxylate-vinyl acetate terpolymers, copolymers comprising ethylene and at least two unsaturated carboxylates, and so on.
Examples of the unsaturated carboxylic acid are acrylic acid, methacrylic acid, etc. Examples of the unsaturated carboxylate are ethyl acrylate, methyl acrylate, 2-ethylhexyl acrylate, stearyl acrylate, glycidyl acrylate, methyl methacrylate, ethyl methacrylate, 2-ethylhexyl methacrylate, stearyl methacrylate, glycidyl methacrylate, etc.
Preferred examples of the copolymer comprising ethylene and at least one vinyl monomer selected from the group consisting of unsaturated carboxylic acids, unsaturated carboxylates and vinyl acetate which are used in the present invention are ethylene-acrylic acid copolymers, ethylene methacrylic acid copolymers, ethylene-methyl methacrylate copolymers, ethylene-ethyl methacrylate copolymers, ethylene-ethyl acrylate copolymers, ethylene-methyl acrylate copolymers, ethylene-vinyl acetate copolymers, ethylene-methyl acrylate-glycidyl methacrylate copolymers, ethylene-methyl methacrylate-glycidyl methacrylate copolymers, ethylene-vinyl acetate-glycidyl methacrylate copolymers, etc.
Percentages of the above vinyl monomer and ethylene in the copolymer may be arbitrarily selected in a range in which the copolymer having the solubility parameter (SP) of 8.45 to 8.70 is obtained. A percentage of the above vinyl monomer in the copolymer is preferably from 5 to 60 wt. %. There is no limitation on a bonding type between the vinyl monomer and ethylene (e.g. random, block, alternating, etc.). A melt flow rate of the copolymer (measured according to JIS K7210 at 190xc2x0 C. under a load of 2.16 kgf) is not limited, and preferably from 1 to 500 g/10 min.
When the composition of the present invention comprises 0.01 to 0.5 wt. part of an organic polysiloxane per 100 wt. parts of the rubber-modified styrenic resin (A), the plane impact strength is further improved. When the amount of the organic polysiloxane exceeds 0.5 wt. part, the plane impact strength will not increase further.
Herein, the organic polysiloxane is intended to mean a compound of the following formula: 
wherein R1 and R2 are the same or different and represent an alkyl group, an aryl group or a phenyl group, and n is an average degree of polymerization and a number of 10 to 1500. The polysiloxane may have an epoxy group, an amino group, a carboxyl group, a vinyl group, a hydroxyl group, a fluorine atom or an alkoxy group at its terminal or intermediate molecular chain.
A structure of the organic polysiloxane to be used in the present invention may be a homopolymer, a random copolymer, a block copolymer or a graft polymer, or the polysiloxane may be one in which a part of the organic groups are substituted by a hydroxyl group, an alkoxy group, a hydroxyalkyl group, etc. Further, two or more organic polysiloxanes may be used in combination.
A kind of the organic polysiloxane to be used in the present invention is not limited. Specific examples thereof are polydimethylsiloxane, polymethylphenylsiloxane, polydiphenylsiloxane, and so on. A viscosity of the organic polysiloxane is not critical. When the viscosity is less than 10 cSt, volatility is too large, while it is larger than 100,000 cSt, it may be difficult to disperse the organic polysiloxane in the composition homogeneously. In general, one having the viscosity in the range from 10 to 100,000 cSt at 30xc2x0 C. is preferred because of easy handling.
A method for the addition of the organic polysiloxane is not limited. For example, the organic polysiloxane can be added in the course of the preparation of the rubber-modified styrenic resin (A), for example, the organic polysiloxane can be beforehand added to the monomer in the polymerization step or added to the polymerization system in the course of polymerization. Alternatively, the organic polysiloxane may be added when the components (A) and (B) are mixed, or when the composition is molded. These methods may be employed independently or in combination.
To prepare the rubber-modified styrenic resin composition of the present invention, determined amounts of the components are dry blended using a mixing apparatus such as a Henschel mixer, a tumbling mixer, and so on, or heated and kneaded at a temperature of 180 to 260xc2x0 C. using a kneading apparatus such as a single or twin screw extruder, a Banbury mixer, and so on, and then the mixture is pelletized. If necessary, additives such as an antioxidant, a heat stabilizer, a UV-light absorber, a lubricant, an antistatic agent, a mineral oil, and so on may be added to the composition.
The injection molded article comprising the rubber-modified styrenic resin composition of the present invention can be produced using a conventionally used injection molding apparatus.
The extrusion molded article comprising the rubber-modified styrenic resin composition of the present invention can be produced using a conventionally used extrusion molding apparatus.
There is no specific limitation on a method for the production of the extrusion molded article by extruding the rubber-modified styrenic resin composition of the present invention. There are exemplified a method comprising melting the resin composition in an extruder and extruding it through a T-die, and a method comprising extruding the molten resin composition in a sheet form from the extruder and then biaxially stretching it by a tenter or an inflation method.
There is no specific limitation on a method for the production of a foam article by foaming the rubber-modified styrenic resin composition. There are exemplified a method comprising melt kneading the rubber-modified styrenic resin composition and a decomposable foaming agent by an extruder and foaming the mixture, a method comprising melting the rubber-modified styrenic resin composition by the extruder, adding a vaporizable foaming agent directly to the composition under pressure in a middle part of a cylinder, kneading them and foaming the mixture, a method comprising impregnating small pellets or beads of the rubber-modified styrenic resin composition with a vaporizable foaming agent in the extruder or an aqueous suspension and foaming the impregnated pellets or beads with steam.
Examples of the decomposable foaming agent are azodicarbonamide, trihydrazinotriazine, benzenesulfonylsemicarbazide, etc. Examples of the vaporizable foaming agent are propane, n-butane, isobutane, n-pentane, isopentane, hexane, heptane, Freons(copyright) etc.
The rubber-modified styrenic resin composition of the present invention finds its applications in an injection molded article field, an extruded sheet field and a foam article filed, in which the properties of the composition are best used. That is, as the injection molded article or extrusion molded article, there are exemplified housings of electronic equipments, business appliances, telephones, office automation equipments, etc. and wrapping materials such as food containers. The foam material is used in the production of wrapping containers by vacuum shaping or pressure shaping. In addition, the rubber-modified styrenic resin composition of the present invention is preferably used as a cushioning material for precision machines, a heat insulator, a construction material, and so on.